The field of the invention is slotted core type optical fiber ribbon cables.
Slotted core type optical fiber ribbon cables have been provided for use in subscriber loops. High fiber count type slotted core ribbon cables must be designed so as to keep the increase in signal attenuation the of optical fibers in the cable within acceptable limits. Such attenuation can be caused by excessive bending of the optical fibers due to forces exerted on the optical ribbons.
Most slotted core type optical fiber cables have either helical or periodically reversing helical slots. When an optical fiber ribbon containing a planar array of optical fibers assumes a helical shape during stranding after being placed in a slot having a helical shape, various strains are placed on the optical fibers in the ribbon as a result of the helical configuration alone. The strain e thereby placed on optical fibers in the ribbon during stranding comprises contributions from the elongation strain .epsilon..sub.e, bending strain .epsilon..sub.b, and twisting strain .epsilon..sub.t. Total optical fiber stranding strain due to these contributions may be expressed as EQU .epsilon.=1/2{(.epsilon..sub.e +.epsilon..sub.b)+.sqroot.[(.epsilon..sub.e +.epsilon..sub.b).sup.2 +(2G.epsilon..sub.t /E).sup.2 ]}, (Equation 1)
where G is the modulus of elasticity in shear and E is the Young's modulus of the optical fiber. Tomita et al., Preliminary Research into Ultra High Density and High Count Optical Fiber Cables, 40th International Wire and Cable Symposium Proceedings pp. 8-15 (1991). Equation 1 does not include strains associated with a tension (or compression) applied to a ribbon as a whole (for example, due to ribbon back-tension during stranding), nor does it include strains introduced during cable installation. As used herein, total optical fiber stranding strain refers to the strain .epsilon. given by Equation 1.
Total fiber stranding strain .epsilon. in a slotted core type optical fiber ribbon cable has been taught to be limited to 0.05% or less. Therefore, a helical slot pitch of 700 mm has been selected in the design of one such cable. In addition to fiber strains during manufacturing, installation stresses of 0.20% are to be allowed for. S. Hatano, Y. Katsuyama, T. Kokubun, and K. Hogari Multi-Hundred-Fiber cable composed of optical fiber ribons inserted tightly into slots, 35th International Wire and Cable Symposium Proceedings pp. 17-23 (1986). The helical pitch is sometimes referred to as lay length.
As illustrated in FIG. 3, the radial distance R.sub.f between the center of the cable O and an end optical fiber in a ribbon is longer than the radial distance R.sub.r between the center of the cable and the central optical fiber(s) in the ribbon, or more precisely, the midpoint of the width spanned by the optical fibers in the ribbon in the plane containing the optical fibers. After stranding, this length difference causes the edge optical fibers in a ribbon to be under tension, as recognized by the prior art.
The same effect causes the central optical fibers in the ribbon to be under compression. The width of the ribbon is a factor affecting the amount of compression, with compression increasing with ribbon width. Ribbon width in turn is determined by the number of optical fibers in the ribbon and the thickness of the coatings on the individual optical fibers. The compression is also a function of the radial distance R.sub.r between the ribbon and the center of the cable, which distance must be at least the radial distance of the slot floor from the center of the cable. As the radial distance of the slot floor from the center of the cable increases, the compression typically decreases. The prior art has not fully taken the compression effect into consideration in cable design optimization.
Slotted core type optical fiber ribbon cables containing ribbons each having a relatively small number of optical fibers have been proposed which have somewhat short slot pitches and somewhat higher total strains. For instance, U.S. Pat. No. 4,826,279 proposed a slotted core type optical fiber ribbon cable having five-fiber ribbons with a slot pitch of 300 mm and a slot floor radius of 3.25 mm. However, slotted core type optical fiber ribbon cables containing ribbons each having greater numbers of optical fibers have been taught to have longer slot pitches. For instance, Japanese laid-open patent publication 62-98313 proposed a slotted core type optical fiber ribbon cable having ten-fiber ribbons with a slot pitch of 550 mm and a slot floor radius of 3.25 mm, which would result in a total fiber stranding strain of less than 0.05%.
The helical length of optical ribbons in slotted core type optical fiber ribbon cable is a function of the slot pitch. Thus, all other factors being equal, a longer pitch helps to reduce the cost of a cable by reducing the length of fiber required.
While the factors listed above would tend to support the design of slotted core type optical fiber ribbon cables having a relatively long helical pitch, three other factors set out below tend to support the design of such cables having a shorter pitch.
First, during cable bending, sections of ribbons on the outside of the bend are under tension, and sections of ribbons on the inside of the bend are under compression. The ribbons tend to move to the region of tension to alleviate strains. A shorter pitch advantageously accomodates such movement.
Second, when a slotted core type optical fiber ribbon cable is bent, forces on the ribbons urge the ribbons to rotate to relieve bending. A representation of such rotation is shown in FIG. 4. The more the ribbons rotate, the deeper the slots must be to contain them, so the outer diameter of the core spacer ribs must be greater. A larger cable is the result. We have found that a shorter slot pitch reduces the amount of ribbon rotation. Therefore, all other factors being equal, a shorter slot pitch advantageously reduces the required cable size.
Third, at low temperatures the plastic material in a slotted core type optical fiber ribbon cable tends to shrink. The ribbons typically shrink less than the rest of the cable, generating excess ribbon length and causing pressure to be exerted on the ribbons. Extra space is required to accomodate this excess ribbon length to avoid such pressures. We have found that a shorter pitch reduces the additional space required to accomodate the extra ribbon length resulting from low temperature conditions. Therefore, all other factors being equal, a shorter slot pitch again reduces the required cable size.
As fiber count increases, diameter minimization becomes more important. Optical fiber cables are typically smaller than electrical cables of comparable message capacity. However, optical cables having a high fiber count typically are larger than already installed lower fiber count optical fiber cables, and duct space is usually at a premium. Larger cables also typically have larger minimum bend diameters and may require larger and more specialized reels and stranding equipment.